The invention relates to fluid measuring devices, and more particularly to continuously operating fluid viscosity measuring devices, such as rheometers.
It is frequently desirable to measure the viscosity of fluids during the course of a chemical process or at other times. The viscosity information obtained from such a measurement may be of immediate or future use. Such data can provide an indication of the quality of a fluid, which can yield information indicative of the extent of a chemical process step, as viscosity is often indicative of a fluid's chemical state. Such information may be used to control the continued operation of the process, especially if the information can be obtained in a timely enough fashion so as to be useful in determining whether the process should continue unaltered or if it should be halted. Alternatively, information related to the quality might be an important aid in determining the future disposition of the liquid, whether such disposition be related to the categorization, grading or determination of the suitability of the liquid for further use.
Information relating to the extent of a process step (such as the extent of chemical conversion or mixing) would be useful in judging the efficacy of a processing step.
In general terms, when liquids which exhibit ideal, or "newtonian" behavior, the viscosity is proportional to the pressure differential across a fluid flow. Thus, by knowing the system variables, the viscosity may be easily derived. However, it is much more common for liquids to exhibit non-newtonian behavior, especially when the liquids are characterized as visco-elastic, such as polymer melts. In the case of non-newtonian liquids, the use of a capillary rheometer apparatus is possible, but calibration measurements for the regime of non-newtonian behavior must be made before the capillary rheometer apparatus can be used to determine viscosity.
In using a capillary rheometer apparatus to determine the viscosity of a liquid exhibiting non-newtonian behavior, data concerning the volumetric flow rate and the pressure differential must be collected. This is because the viscosity is a function of the shear velocity, which by application is a function of the volumetric flow rate, "V", through the capillary. In order to be effective, for non-newtonian liquids, the pressure differential or drop, resp., must be varied over the largest possible range in order to precisely determine the viscosity function. This requires that the volumetric flow rate be varied over a broad range, preferably over several orders of magnitude so as to provide useful data. This is especially desirable because differences in the viscosity of two chemically similar materials is higher for small shear rates than at higher shear rates. This relationship is especially true in the case of high polymer melts.
If the pump in the rheometer apparatus should operate by a constant rotational rate or a constant pressure to determine the viscosity, then it should operate under the conditions of lower shear forces. To achieve this, the liquid to be characterized is first transferred from the reactor by a connection means, typically a tube or pipe, to the rheometer apparatus which comprises a pump and a capillary. From the rotational rate of the pump, the volumetric flow rate of the liquid may be established, and from the measured pressure drop across the capillary, data may be collected from which the viscosity may be determined. Two data collecting methods are available.
The first method requires that the pump be maintained at a constant rotational speed, which provides a constant volumetric flow rate through the capillary, and that pressure drop data over the capillary be collected. For generating this data, the pump's rotational speed is varied between discrete rotational rates over ranges which should encompass three or four orders of magnitude. For example, one such range might be between 0.1 RPM and 100 RPM, a variation on the order of 1000, or three orders of magnitude, where the slowest rotational rate is onethousand times smaller than the fastest rotational rate. This first method is the one most commonly used for rheometric measurements.
The second method of collecting the data requires the generation of a constant pressure drop over the capillary, and is the most frequently used method for the determination of the viscosity of plastic melts. A constant pressure upon the liquid may be achieved, for example, through the use of a constant mass loaded upon a test cylinder. In accordance with the aforementioned calibration of the capillary, a constant internal shear stress is generated in the fluid. The dependent value in the measurement process here is the volumetric flow rate. The appropriate measuring instrument here is the so-called "melt indexer" and its measuring procedure is a process which is standardized worldwide. The value determined from the measurement process, the "melt flow index" (MFI) is determined for a sample of the discharged liquid for a ten minute interval. The units of measurement of the melt flow index are g/10 minutes, or cm.sup.3 /10 minutes. For the analogous measurement with the use of a metering pump, it is required that the rotational speed of the pump be specifically controlled to establish a constant pressure differential, where the rotational rate is varied across four orders of magnitude.
In the field of capillary rheometers for the continuous measurement of viscosity of liquids in reactors, mixers, extruders or other such process vessels or devices, it is necessary to use a metering pump for withdrawing the liquid from the reactor or the like and pressuring it through the capillary. By this means, the pressure differential over the length of the capillary will be determined. The liquid will may thereafter be allowed to exit (as in a bypass rheometer), or it may be returned to the reactor (as in a parallel flow rheometer). In either form, the rheometer represents a closed system whose overall through-put will be established by the metering pump.
The viscosity of many fluids depends not only on the shear rate but also on the hydrostatic pressure. In case of parallel flow rheometers servicing variably pressured reactors, the viscosity measurement must be decoupled from the pressure within the reactor. To eliminate this pressure dependency, one may provide a second over-stepping pump downstream of the capillary and activate it, wherein the output duty of the second pump is greater than that of the metering pump, and which reduces the output pressure of the capillary to or near zero. One such system of this type is known from U.S. Pat. No. 3,548,638 to Uchida, et al. for an "Apparatus and Method for Continuously Determining Viscosity".
Nevertheless, with both of these arrangements, those with or without the second pump, the rotational rate of the pumps must be variable over a range of several orders of magnitude as the flow rate through the rheometer's capillary and connecting means is equally large and proportional to the rotational rate of the metering pump. With the change of the rotational speed, the volumetric flow rate varies in proportion to the rotational rate of the metering pump, which must be changeable over many orders of magnitude. With the change in rotational speed, for example, the average residence time varies inversely with the volumetric flow rate. For example, if the rotational speed is reduced by a factor of 500, then the residence time increases 500 fold. A speed reduction by a factor of 100 would lead to an increase in residence time by a factor of 100. The specific volumetric flow rate of rotary pumps lie about between 0.5 cm.sup.3 /rpm and 3.2 cm.sup.3 /rpm. Typical maximum long term rotational speeds of rotary pumps are approximately 100 rpm.
One known embodiment uses about 40 cm.sup.3 of fluid in a sidestream capillary rheometer between the test port and the end of the capillary. The metering pump supplies 0.65 cm.sup.3 /rpm. To cover the range of a normally occurring melt index range (DIN 53 735; 0.1=&lt;MFI=&lt;50) at a constant pressure differential, the rotational speed must be variable at least between 100 rpm and 0.2 rpm. This would lead to a range of the intermediate residence time from approximately 37 seconds to over 5 hours, which not only shows, that this range cannot be controlled, but rather also that, the control of the variation of the melt in a narrower range during the process or reaction for continuous process control is not possible, or is, at best, highly imprecise. Further, the measurement should not occur long after the sampling, as the polymer melt is usually not sufficiently stable to bear a high process temperature for a very long time without a change in its molecular structure (thermal degradation).
Minimally, to obtain somewhat useable results where there are no large variations in the viscosity, one chooses the smallest possible transport volume through the capillary by, for example, direct attachment of the capillary rheometer to the chemical process vessel. But this direct attachment is not without attendant difficulties, and is of limited use because there are problems associated with the handling and/or heating or cooling of a rheometer so attached. It has also shown itself, that the suction volume before the metering pump is always at least ten times larger than the volume of the capillary and the volume of the metering pumps (2 to 5 cm.sup.3). The transport time through this necessary section of the rheometer apparatus is therefore ten times longer than the actual measuring time during which the fluid flows through the capillary. Therefore, a small residence time may be achieved only through the optimized construction of the rheometer and its immediate connection to the chemical process vessel, via the testing port. Nonetheless, under these circumstances, the establishment of a smaller ratio is generally not possible. This means that the response time of the rheometer is determined mainly by the connecting pipe and not by the rheometer.
With a newtonian liquid, the shear velocity of equivalent volume streams is inversely proportional to the third power of the capillary's inner diameter or measure of internal annular cross section. After the minimization of the volume of the connecting pipe, the expansion of the capillary's inner diameter or measure of internal annular cross section is a further known method to minimize the necessary residence time of the liquid passing through the rheometer apparatus, as this assures minimal residence time by small shear vector. However, this means that liquids with relatively large melt indices can no longer be measured because first, the rotational speed of the metering pump would be exceeded, and second, energy transmitted through the pump to the liquid would cause a transition to impermissibly high temperatures. In order to measure liquids having relatively large melt indices, the capillary used must be provided with a varying diameter. The substitution of a first capillary with a second capillary having a different diameter is not possible during the control of a process, because the time required to effect this change with presently known capillary rheometer arrangements requires that the capillary be once again recalibrated by use of a liquid with known viscosity for use with non-newtonian liquids.
Thus, from the foregoing discussion it is clear that here remain many problems in the art dealing with the viscosity determination of liquids, especially for fluids which exhibit non-newtonian behavior, including those fluids known to the art as polymer melts. The art therefore requires an improved apparatus for use in the determination of liquid viscosity, particularly for polymer melts whose operating characteristics exhibit an advance over devices known to the prior art.